As a technique for backing up data of a volume managed by a storage system, there has been conventionally known a method of managing a snapshot of a state of the volume at a predetermined time point.
In the case of managing a snapshot, when a write access to a save source volume (primary volume) occurs, data in a write-destination area of the primary volume is saved into a save-destination volume (secondary volume). After that, write-target data is copied to the primary volume, and a response is returned to a write-access source. Therefore, there is a problem that it takes a long time to return the response to the write-access source after the occurrence of the write access, and write response is bad.
In comparison, there is known a copy-after-write (CAW) technique in which write response is speeded up by, when a write access occurs, writing write data into a cache memory, returning a response, saving data of a primary volume into a secondary volume in asynchronization with the write access, and writing the write data of the cache memory into the primary volume (for example, see Patent Literature 1).